It is often desirable to determine whether an image has been retouched or altered from its original form. For example, if a photograph is offered as evidence in a legal proceeding, it is important to know whether the photograph has been modified from the original. In digital image processing, this process has been referred to as "image authentication" (see "Image Authentication for a Slippery New Age" by Steve Walton, Dr. Dobb's Journal, April, 1995, page 18 et seq.). The approach describe by Walton envisions adding checksum bits directly to the digital image so that if any pixel(s) is changed, the checksum will change thus identifying the image as having been modified. Although useful, this approach suffers from several shortcomings. If the image has been printed and then redigitized, the checksum information is lost so that further modifications of the image cannot be detected. The addition of even a small amount of noise to the digital image or resizing, cropping, tone scaling, or rotation of the image frustrates the operation of the technique. Further, the technique described by Walton only indicates that some change has been made to the image, but does not show where or what changes have been made. It would be desirable therefore to provide an improved digital image processing method for detecting when and where an image has been retouched or modified.